Transcriptome analysis of the binucleate ciliate Tetrahymena thermophila with asynchronous nuclear cell cycles

Abstract

As a prototypic ciliated protozoan, Tetrahymena thermophila harbors two functionally and physically distinct nuclei within a shared cytoplasm. During vegetative growth, the cell cycles of the diploid germline micronucleus and polyploid somatic macronucleus are offset. Micronuclear S phase initiates just before cell division and is completed in daughter cells prior to the onset of macronuclear S phase. Whereas mitotic micronuclear division occurs mid-cell cycle, amitotic macronuclear division immediately precedes cytokinesis. Here we report the first RNA-seq analysis across the cell cycle of a binucleated organism. RNA was isolated at 30 min intervals across 1.5 vegetative cell cycles, starting with a macronuclear G1 population synchronized by centrifugal elutriation. Using MetaCycle, 3244 of the predicted 26,000+ T. thermophila genes were shown to be cell cycle regulated. Proteins that are required in micro- and macronuclei exhibit a single mRNA peak that correlates with their macronuclear function, while the expression of nucleus-limited protein-coding genes, including nucleoporins and importins, peak prior to their respective nucleus-specific role. Cyclin D and cyclin A/B genes showed distinct expression patterns that predict nucleus-specific functions. Clustering of periodically expressed genes revealed seven gene expression patterns. Four clusters have known PANTHER GO biological processes that are overrepresented for G1/S and G2/M phase functions. We propose that these clusters encode known and novel factors that coordinate micro- and macronuclear-specific events such as mitosis, amitosis, DNA replication and cell division.